Gates was born in Bremerton, Washington, and enlisted in the US Army after graduation from high school. He served in the army and fought in World War II. In November of 1946 he was honorably discharged from the army, and he enrolled in the University of Washington under the G.I. Bill, where he earned a Bachelor's degree in 1949 and a law degree in 1950. He married Mary Maxwell Gates, whom he met at UW, and who died in 1994. In 1996 he married Mimi Gardner Gates, who is director of the Seattle Art Museum. With her he had three children, Kristi, Bill, and Libby. He practiced law until 1998, primarily at the company now known as Preston Gates & Ellis (PGE), which he co-founded as Shidler & King in 1964.
In 1998, Gates retired from PGE. He currently serves on the Board of Regents for the University of Washington, and runs the Bill and Melinda Gates Foundation.